Multimode interference (MMI) couplers are known optical couplers that include one or more input waveguides (a waveguide typically comprises a waveguide core surrounded by a cladding material that has a lower refractive index than the core material) that are coupled to a wider waveguide section (also called a multimode or MM section), which is in turn coupled to one or more output waveguides. An MMI coupler allows one or more input signals having different wavelengths to be split or demultiplexed, combined and/or modified into one or more output waveguides through the self-imaging effect.
Symmetric 1×2 or 2×2 MMI couplers may have one or two inputs and two outputs, and the optical signals supplied from each output are typically designed to have the same or substantially the same power. Accordingly, symmetric 1×2 and 2×2 MMI couplers may be designed to have a 50:50 power splitting ratio. A multiplexer including cascaded symmetric MMI couplers may be provided to combine optical signals. Such multiplexers may be limited to multiplex 2n input optical signals (n being integer), each of which being carried by a corresponding waveguide to the multiplexer. The coupling loss (i.e., fraction of power lost) per optical signal associated with symmetric MMI based optical multiplexers may have a theoretical minimum equal to (1−1/n) when combining a number of optical signals equal to a power of 2 (e.g., at 4, 8, 12 4, 8, 16, etc.). Symmetric MMI couplers have broadband performance, i.e., such MMI couplers may be employed to multiplex or demultiplex optical signals over a wide range of wavelengths and are relatively compact in size as compared to combiners based on arrayed waveguide gratings (AWG).
Accordingly, MMI couplers and splitters are generally used for 2n waveguide configurations, and, as such, the number of waveguide inputs into the multiplexer or the number of outputs from the demultiplexers is equal to 2n where n is an integer. That is, the number of input waveguides may be 2, 4, 8, 16 . . . . Some optical circuits that are integrated on a substrate, e.g. photonic integrated circuits or PICs, however, multiplex or demultiplex a number of optical signals, other than a power of (2n). For example, in certain configurations, a multiplexer may be required to combine ten optical signals, each of which being supplied to the multiplexer by a corresponding one of ten input waveguides. Conventionally, arrayed waveguide gratings have been employed in order to multiplex or demultiplex a non-power of 2 number of input optical signals. AWGs have minimal insertion loss, but are configured to combine or split only certain wavelengths, and thus have strict spectral limitations. Accordingly, broadband multiplexers or demultiplexers that combine or decombine a non-power of 2 number of optical signals with minimal insertion loss is desired.